[hard as nails] {adj. phr.}, {informal}
1. Not flabby or soft; physically very fit; tough and strong.
* /After a summer of work in the country, Jack was as hard as nails, without a pound of extra weight./
2. Not gentle or mild; rough; stern.
* /Johnny works for a boss who is as hard as nails and scolds Johnny roughly whenever he does something wrong./